Optical range discriminator for laser tv camera

ABSTRACT

An optical beam scanner having a pair of polygonal cylinders with exterior mirror faces to sweep a continuous incident laser beam between particular angular limits in one direction and between particular angular limits in the transverse direction, simulating line-by-line and frame-by-frame scanning. Reflections scattered back from objects in the path of the swept beam and incident to the line-by-line scanner are redirected by the lineby-line scanner toward a photomultiplier assembly which is angularly adjustable about the line-by-line scanner for range discrimination. A television monitor, synchronized with the scanners and intensity modulated by the photomultiplier output, images any reflecting object at the selected range and within the angular sweep limits of the beam.

I United States Patent [151 3,644,666

Green 1 Feb. 22, 1972 541 OPTICAL RANGE DISCRIMINATOR 5,445,555 5/1969Kahn .555/5 FOR LASER TV CAMERA 3,442,193 5/1969 Page] ..356/4 [72]Inventor: Milton Green, 980 Flanders Road, Mystic, primary Examine,R0be1-t Richardson Conn- 06355 Assistant Examiner-Joseph A. Orsino, Jr.5 Attorney-Richard S. Sciascia, Louis B. Applebaum and [22] Filed. July7, 1970 Arthur L. Bowers [21] Appl. No.: 52,874

[57] ABSTRACT [52] US. Cl. ..178/6.8, 178/7.6, 356/4, An optical beamscanner having a pair of polygonal cylinders 356/ 17 with exteriormirror faces to sweep a continuous incident laser [51] Int. Cl ..G0lc3/08, H04n 7/18 m ween particular angular limits in one direction and[58] Field of Search ..356/l, 3, 4, 5, 15, 16, 17; between particularangular limits in h transverse irec i n,

173/63 7 6 simulating line-by-line and frame-by-frame scanning.Reflections scattered back from objects in the path of the swept 5References Cited beam and incident to the line-by-line scanner areredirected by the line-by-line scanner toward a photomultiplier assemblyUNITED STATES PATENTS which is angularly adjustable about theline-by-line scanner for range discrimination. A television monitor,synchronized 3516743 6/1970 with the scanners and intensity modulated bythe photomul- 3'443870 5/1969 tiplier output, images any reflectingobject at the selected L574 5/ 1939 range and within the angular sweeplimits of the beam. 3,372,230 3/1968 3,555,178 l/l97l Humiston ..356/5 2Claims, 4 Drawing Figures ELEV/.510 Man/17w? l5 PAIENTEDFEB 22 I972SHEET 3 BF 3 INVENTOR. M/L TON GREEN BY M of M 7 ,JI'IV OPTICAL RANGEDISCRIMINATOR FOR LASER TV CAMERA This invention described herein may bemanufactured and used by or for the Government of the United States ofAmerica for governmental purposes without the payment of any royaltiesthereon or therefor.

BACKGROUND OF THE INVENTION Continuous-wave laser systems have beenknown for use as viewing aids where direct visual observation is partlyor wholly obscured by fog, or rain, and in darkness wh'ere clandestineobservation is required. In such systems known in the art, a laser beamis swept line-by-line and frame-by-frame by a dual orthogonal polygonalmirrored cylinder scanner. Some of the laser beam energy reflected bydiscontinuities in the beam path is directed back toward the scanner. Inthe vicinity of the scanner, a photomultiplier equipped with a sharplytuned optical filter detects all incident reflected laser beam energy. Atelevision monitor having he same line and frame scan rates as the sweptlaser beam is connected to the photomultiplier circuit, so that beamintensity in the television monitor is continuously related to the levelof reflected laser energy sensed by the photomultiplier. All objectswithin range scanned by the laser beam and which scatter laser lightback to scanner appear on the television monitor. There is no rangediscrimination. Also, resolution is poor because of beam spreading,stray reflections returned by random discontinuities in the propagationmedium and by multiple bounce. Since energy is scattered back fromobjects anywhere in the entire range of the laser beam and within theangular scan limits, the noise level on the television screen masksimages of interest. The range of a viewed target cannot be ascertainedfrom such equipment.

SUMMARY OF THE INVENTION This invention employs a laser, amechanical-optical scanner, a photomultiplier assembly equipped withfocusing lens and iris and sharply tuned optical filter, plus televisionmonitor in an arrangement wherein the photomultiplier assembly sees onlyreflections off the line-by-line scanner, and at a particular angle,instead of all laser light scattered back from objects in the path ofthe propagated laser beam. The photomultiplier assembly is adjustableabout the line-by-line scanner so that only that portion ofscattered-back laser light returned from a selected range and directionis sensed by the photomultiplier. The focusing lens and iris function todirect to the photomultiplier only that laser light energy which arrivesat the photomultiplier from the line-by-line scanner at that anglecorresponding to looking at a particular range. The iris excludesextraneous rays arriving from slightly different angles and thus limitsdepth of field and sharpens the image on the television monitor. Fordirectional discrimination, the equipment, except for the televisionmonitor, is mounted on a platform for rotation in azimuth and fortraining in elevationdepression.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are top and side viewsrespectively of an embodiment of this invention shown diagrammaticallyand including ray directions propagated and returned;

FIG. 3 shows an arcuately adjustable support for the photomultiplierassembly; and

FIG. 4 illustrates the operation of the horizontal scanner.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIGS. 1 and 2 there is showna mechanical-optical scanner for propagating and sweeping a thin lightbeam viz about 1.5 millimeter diameter in a manner that simulates thedeflection of the electron beam of a television receiver's cathode-raytube in response to the influence of its horizontal and verticaldeflection means. The mechanical-optical scanner deflects the thin lightbeam 10, emitted by a continuous beam optical laser 11, line-by-line andframe-by-frame as in a television receiver. Two polygonal cylinders 12and 13 each having outer mirror faces and radial symmetry are mounted inbearings 14 and 15 respectively for rotation about orthogonalnonintersecting axis. The shafts l6 and 17 of the mirror cylinders aredriven by precision, adjustable speed motors, not shown. The laser beamis directed to a fixed position small mirror 18 that is large enough toaccommodate the beam, and small enough not to interfere with reflectedrays. Mirror 18 redirects beam 10 toward the axis of mirror cylinder 12for horizontal scanning or line-by-line scanning. In FIG. I, the changein beam direction introduced by mirror 18 is and in FIG. 2, theredirected beam describes an angle of incidence of about 5' of thenormal to the axis of cylinder 12, the direction of the beam from mirror18 and the axis of cylinder 12 are coplanar. The beam strikes the end ofthe cylinder mirror which is in position 19 in FIGS. 1 and 2. When thecylinder 12 is rotated, the laser beam brushes each mirror face insuccession and as the beam spot moves across the width dimension of eachmirror face of cylinder 12, the reflection of the laser beam sweepsthrough an angular range which is twice the angle subtended by one faceof cylinder 12. viz 2 (360/i n) where n equals the number of faces ofcylinder 12; the limits of the angular range are indicated by a and c inFIG. 1; ray b indicates the intermediate direction. When the beam movesoff the edge of one mirror and onto the edge of the succeeding mirror ofrotating cylinder 12, the beam reflection flies back as in line-by-linetelevision scanning.

An elongated planar mirror 20 is fixedly supported in the path of theswept beam from mirror cylinder 12 as shown in FIG. 1 to redirect theswept beam to the mirror position 21 of mirror cylinder 13 as shown inFIG. 2 where it describes a linear path along mirror 21. When the mirrorcylinder 13 also is rotated, the horizontally swept beam is deflectedalso through an angular range x-z that is orthogonal to the otherangular range a-c. The beam direction y is intermediate the angularrange x-z. The angular range x-z is oblique relative to the axis ofcylinder 12.

The rate of rotation of mirror cylinder 12 is very much greater thanthat of mirror cylinder 13. However, the horizontal sweep rate of theembodiment as designed for use in air can be much slower than thehorizontal sweep rate if the embodiment is designed for use underwater.The attenuation underwater limits the range to several hundred meters,and in many situations to less than meters; therefore the elapsed timebetween propagation of the beam and return is very short. In air theeffective range is incomparably greater using an optical laser of thesame power so that the time elapsed between propagation and return fromthe very much longer useful range is much greater.

The swept beam is propagated outward from mirror cylinder 13. Portionsof the propagated beam energy are intercepted by objects in the path ofthe beam. Objects assumed along directions xy, and z respectively,scatter energy back; those rays intercepted by mirror 19 of cylinder 12are indicated as rays 1, y, and z, respectively. When energy isscattered back by any object, the returned rays fill the mirror facebecause the cross section of the scattered energy returned to thescanner is much greater than the cross section of the propagated beam.Reflected rays x or reflected rays y or reflected rays z representreflections that might be returned from any direction within the sweepangle x-z. Illumination of the entire mirror 19 is represented byidentically designated rays intercepted by the top T and B of the mirror19 in FIG. 2. The plural rays x", or y" or z" are reflections of ray x,y and z by mirror 19. The identically designated rays are parallel andare intercepted by collecting lens 22 mounted in holder 23 that nests aphotomultiplier 24. An optical filter 25 preferably of the type known inthe laser art, sharply tuned to the frequency of the optical laser issupported in front of the photomultiplier to reject radiation offrequencies other than that of the laser. In the case of deep underwateroperation or elsewhere the laser light is the only significant source ofradiation likely to be incident to the scanner cylinder, or in othersituations where no disturbing light sources are present, the sharplytuned filter may be removed to dispense with filter attenuation of theincoming signal.

A fixed vertical slit or an adjustable vertical slit 26 is mounted inthe housing in front of the filter and operates to minimize extraneousrays. This allows for flexibility in obtaining best picture resolution.If the slit is adjustable depth of field may be varied. The rays x", y",and z" are brought to focus at the slit aperture. The lens is adjustablerelative to the slit; for this purpose the housing parts are shownthreadedly engaged. Any suitable focusing arrangement may be used. Theangle in FIG. 1 between a propagated beam d and the ray D, which is thatpart of the energy of beam (I returned to the scanner l2 and againreflected by the same mirror face of the rotating scanner; the angle 0is a measure of the range of the object. The lens and photomultiplierholder 23 is supported by a bracket 27 selectively adjustable along acalibrated arcuate range scale 28 whose axis extends through point R ofthe cylinder 12 and is parallel to the cylinder axis. The bracket 27carries an indicator pointer 27 a.

The range is selected by angularly positioning the opening or entranceslit of the photomultiplier assembly along the arcuate scale bymanipulating the bracket 27. The bracket axis passes through point R.

A television monitor 29 serves to display an image of any object thatintercepts the beam propagated by the scanner assembly. Horizontal andvertical sweep circuits of the television monitor are set to correspondto the product of rate of rotation and number of mirror faces of thehorizontal and vertical scan cylinders 12 and 13 respectively. Aphotodiode 30 in FIG. 1 and a photodiode 31 in FIG. 2 along the limitsof.

horizontal and vertical sweep angles deliver synchronization signals tothe television monitor. The electron beam of the television monitor isintensity modulated in accordance with the output of photomultiplier 24which is coupled by a video amplifier 32 to the intensity controlportion 33 of the television monitor. Two or more ranges can be observedsimultaneously by using two or more television monitors plus aphotomultiplier assembly for each. at different range positions alongthe same arcuate indicator 28. Alternatively the arcuate indicator maybe omitted and instead the scanner assembly may be provided with severalfixed photomultiplier assemblies distributed arcuately whereby eachlooks at another range. A selector switch may be used to couple theoutput of one photomultiplier to the television monitor if more than onemonitor is not feasible.

FIG. 4 illustrates the interaction of the horizontal scanner and thelaser beam. An octogonal cylinder 34 rotates about its axis at angularvelocity (0,. A laser 35 emits a beam 36 which is bounced off mirror 37to one end of mirror face 38 of the scanner, which in turn reflects theincident beam as beam 39, corresponding to the relationship shown inFIG. 1. The vertical scanner shown in FIGS. 1 and 2 is omitted. When theoutgoing beam 39 encounters an object and is scattered a portion of theenergy retraces the path of beam 39 after an elapsed time t. In the timeI, the horizontal scanner has rotated through angle til to theorientation shown in broken lines. The returned energy is reflected bythe same face 38 of the arcuately displaced scanner. If the scanner werenot rotating, the reflection returned along direction 39 would bebounced off the mirror face along the direction 36. Since the angle ofincidence of the returned energy at mirror face 38 is increased by angle0, the angle of incidence plus the angle of reflection is increased by20. Therefore returned beam 39 is bounced off mirror face 38 alongdirection 40 which is at angle relative to the angle of the originalbeam 36 incident to the scanner. By locating the lens andphotomultiplier assembly at the arcuate position displaced 20 from thebeam direction 36, reflections of any object at a range which would betraversed twice by the laser radiation in the time cylinder 34 rotatesthrough angle 0 is directed to the photomultiplier and essentially allunwanted scattered rays from other than the selected range to not reachthe photomultiplier.

The range is measured from point R. Point R is not fixed. As thecylinder rotates, it shifts between narrow limits. The axis about whichthe photomultiplier assembly is adjusted is established through the meanlocation of R. The mean location of R is determined as the intersectionof beam 36 with mirror face 38 when angle 45 between the reflecting faceof the mirror and the plane defined by beam 36 and the axis of rotationof the cylinder 34 such that the following equation is substantiallysatisfied:

N is the number of mirror faces of the cylinder. This position of pointR is essentially the mean point of excursion of the intersection of thelaser beam with the cylinder.

The vertical scanner moves so much more slowly than the horizontalscanner, it can be desirable to use a single mirror face rather than afull cylinder and drive that mirror arcuately to describe an arcuateexcursion equal to that which would be achieved by a fullframe-deflection cylinder and to quickly return the mirror from the endof an excursion to its starting position.

The entire assembly is mounted on a training platform not shown fordirecting the propagated beam.

This invention extends the range of optical observation under difficultconditions as compared to other optical systems known in the art.Because it can focus on a selected range and in a selected direction andbecause it can reject noise due to light scattering it presents a goodimage on the television receiver when direct observation is very poor orimpossible. It is useful as a visual landing aid for aircraft in foggyand inclement weather because it will penetrate fog to a far greaterrange than any ordinary optical system and with a retroreflectorequipped target it will penetrate fog to even a still greater range. Itis useful for ships at sea maneuvering in fog darkness and by landvehicles proceeding through fog or in darkness and for underwaterexploration and reconnaissance when provided with a laser emittingblue-green light. In fact, for short range submarine sighting andunderwater navigation, it is an ideal complement to sonar particularlyin hostile environments where silence is essential. detection isvirtually impossible.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

Iclaim:

l. A mechanical optical scanner comprising two adjacent polygonalcylinders each having planar mirrored outer faces and having radialsymmetry,

means mounting the two cylinders so that their axes are orthogonal andnonintersecting,

an optical laser for emitting a continuous beam,

a mirror in the emission path of the optical laser to deflect the beamtoward one end of one of said cylinders so that the deflected beam. iscoplanar with the axis of that cylinder and is at a small angle ofincidence relative to the normal to the axis of that cylinder to reflectfrom the cylinder laterally outward of the cylinder and,

a planar mirror positioned relative to the two cylinders to deflect aswept beam from said one cylinder longitudinally along the surface ofthe other cylinder,

a photomultiplier assembly including a housing, optical lens, slit,optical filter sharply tuned to the laser frequency and photomultiplierfor sensing laser beam energy returned from objects in the path of thepropagated beam and bounced off said one cylinder,

means supporting said photomultiplier assembly for arcuate adjustmentabout an axis essentially parallel to the axis of said one cylinder andpassing through a median point of intersection of the laser beam fromsaid first mirror and respectively for delivering sync signals to thedeflection the surface of said one cylinder, circuits of the televisionmonitor,

a television monitor having deflection means for operating h b h l i imonitor presents an image f h at the same rate as the laser beam sweeprates produced which is located at a range selected by arcuatepositioning of the photomultiplier assembly 2. A mechanical opticalscanner as defined in claim 1 further including a range-calibratedarcuate scale along the by the cylinders, 5 means for vaying electronbeam intensity in the television monitor in accordance with the signallevel from the photomultiplier, two photodiodes in the path of the beamat one limit of its adjustment path of the photomumpher assembly sweeprange as propagated by each of the cylinders 10

1. A mechanical optical scanner comprising two adjacent polygonalcylinders each having planar mirrored outer faces and having radialsymmetry, means mounting the two cylinders so that their axes areorthogonal and nonintersecting, an optical laser for emitting acontinuous beam, a mirror in the emission path of the optical laser todeflect the beam toward one end of one of said cylinders so that thedeflected beam is coplanar with the axis of that cylinder and is at asmall angle of incidence relative to the normal to the axis of thatcylinder to reflect from the cylinder laterally outward of the cylinderand, a planar mirror positioned relative to the two cylinders to deflecta swept beam from said one cylinder longitudinally along the surface ofthe other cylinder, a photomultiplier assembly including a housing,optical lens, slit, optical filter sharply tuned to the laser frequencyand photomultiplier for sensing laser beam energy returned from objectsin the path of the propagated beam and bounced off said one cylinder,means supporting said photomultiplier assembly for arcuate adjustmentabout an axis essentially parallel to the axis of said one cylinder andpassing through a median point of intersection of the laser beam fromsaid first mirror and the surface of said one cylinder, a televisionmonitor having deflection means for operating at the same rate as thelaser beam sweep rates produced by the cylinders, means for vayingelectron beam intensity in the television monitor in accordance with thesignal level from the photomultiplier, two photodiodes in the path ofthe beam at one limit of its sweep range as propagated by each of thecylinders respectively for delivering sync signals to the deflectioncircuits of the television monitor, whereby the television monitorpresents an image of that which is located at a range selected byarcuate positioning of the photomultiplier assembly.
 2. A mechanicaloptical scanner as defined in claim 1 further including arange-calibrated arcuate scale along the adjustment path of thephotomultiplier assembly.